Decompression engine brakes can be used as auxiliary brakes, in addition to wheel brakes, on relatively large vehicles, for example trucks, powered by heavy or medium duty diesel engines. A decompression engine braking system is arranged, when activated, to provide an additional opening of an engine cylinder's exhaust valve when the piston in that cylinder is near a top-dead-center position of its compression stroke so that compressed air can be released through the exhaust valve. This causes the engine to function as a power consuming air compressor which slows the vehicle.
In a typical valve train assembly used with a decompression engine brake, the exhaust valve is actuated by a rocker arm which engages the exhaust valve by means of a valve bridge. The rocker arm rocks in response to a cam on a rotating cam shaft and presses down on the valve bridge which itself presses down on the exhaust valve to open it. Decompression engine brake systems can be based on an actuation capsule, assembled on the rocker body and directly acting on the valves or the valve bridge. For such systems, the cam is usually designed with a total shape, resulting from the sum of the engine brake lift and the normal valve opening (that used for positive power mode). The total cam lift is then provided with an additional closing ramp, to let the valve train back from brake lift to base circle.
During positive power mode, a dedicated lost motion system excludes the engine brake lift, so only a net valve lift (i.e. normal valve opening) is provided. On brake mode, with a proper actuation pressure level (such as may be regulated by a solenoid valve), the capsule may assume a designed working position, in order to exclude the lost motion system and modify the lift shape of exhaust valves, thus anticipating the valve opening and enabling cylinder decompression.
When the capsule is enabled for engine braking, a late closing of braking valves may occur because of residual cam closing ramp extension, which is usually hidden by the lost motion system during positive power mode. Late valve closing during engine braking is usually tolerated, as the exhaust closing stage has a relatively low impact on the braking power. For engine configurations provided with a floating bridge on two exhaust valves, and being designed to use only one valve for braking (in order to reduce the pressure load on the valve train by half), on anticipated opening only one valve moves, while the other is kept closed. As a consequence, the bridge tilts proportionally to the brake valve lift, until the lost motion of the system is completely recovered.
Such behavior is desirable at opening, while a closing delay of brake valve should be avoided, as the two valves should be desired to close at the same time and at the designed speed. Alternatively, if the bridge tilts also at closing stage (for example because of the engine brake capsule still expanded or difficult to compress), the engine brake valve closes last and normally seats, while the slave valve (that closes first) comes to the seat insert at a double velocity with respect to design with subsequent risks of failure at long durability. It is desirable to provide a system where both exhaust valves correctly close.
The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.